Over recent years there have been developed various compact arc discharge lamps and gas discharge lasers. Broadly, such devices comprise a tubular quartz envelope having at least two metallic seals composed of a refractory metal associated therewith, said seals being typically composed of molybdenum or tantalum. Electrode rods, which are typically composed of tungsten, are carried from the metallic seals into the quartz envelope in a manner suitable to maintain the hermetic seal thereof. The quartz envelope of the device is filled to a pressure of up to about 14 atmospheres (1,418,550 Pa) with a noble gas, such as xenon, argon or radon and, typically, one or more condensible metals, halogens and/or metal halides such as the halides of tin, mercury, scandium, iron, sodium, thallium, indium and rare earth metals. The principal role played by the halogen or halide components is to establish a transport cycle mechanism for the metal or metalloid values within the envelope. The practical benefits of the metal halide components contained or found within the envelope are to preserve electrode life and optical clarity of the quartz envelope and also to tailor the light emissions to the desired wavelengths. In operations, an arc is struck across the electrodes of the device, thereby generating intense luminosity and vaporizing condensible materials contained or formed within the envelope. As the environment within the quartz envelope heats the resistance across the arc normally diminishes. Accordingly, a current limiter is usually placed in circuit with the device in order to properly adjust the current in response to such resistance changes.
While devices of the compact arc discharge lamp and gas discharge laser type possess the considerable advantages of intense light output of controlled wavelengths at high efficiencies, nevertheless they are also possessed of several considerable disadvantages which have mitigated against more widespread applications thereof.
Firstly, it is desirable that operations of such devices be undertaken at internal temperatures sufficiently high as to prevent condensation of the condensible components thereof, particularly the halides, upon the refractory metal seals of the quartz envelope. This is so because said condensible components are often found to be quite corrosive with respect to refractory metal seal materials such as molybdenum or tantalum when contacted therewith at elevated temperatures. This internal corrosion, of course, can lead to short service life of the device and, moreover, failure of the refractory metal seals during operations under the relatively high internal pressures involved can be of a catastrophic nature. Thus, compact arc discharge lamps and gas discharge lasers are desirably operated such as to maintain the seal portions thereof at temperatures in excess of the condensation temperatures of the metal and/or metal halide components contained within the envelope, say, on the order of above about 400.degree. C.
On the other hand, the refractory metal seal elements of such discharge devices generally are susceptible of oxidative degradation when contacted with an oxygen-containing atmosphere at elevated temperatures. For instance, molybdenum, a typical refractory metal material, oxidizes excessively rapidly in air at temperatures of above about 350.degree. C. Thus, it is often found that maintenance of high temperatures at the seal portions of the device, while desirable from the standpoint of avoidance of condensation of metals and metal halides and internal corrosion of the metal seals, is in fact not practicable when considered from the standpoint of external oxidation of said metal seals. In view of the foregoing competing interests, therefore, it is normally the case that discharge devices of the foregoing type are usually operated under compromised conditions such that only some portions thereof are maintained at desirably high temperatures. For instance, the metal seal-containing portions of such devices are often externally cooled during operations in order to avoid excessive external oxidation of the seals. This concession to external oxidation, however, fosters the condensation and internal corrosion problems mentioned previously and can also generate mechanical stress problems arising from the thermal coefficient of expansion mismatches inherently existing as between the quartz envelope and the metallic seal and electrode elements of the construction.
These mechanical stress problems are generally addressed in the prior art by means of various complex glass-to-metal press seal constructions of the multiple ribbon, rod or graded types whereby several different glass compositions of intermediate thermal coefficients of expansion are interposed between the low coefficient quartz envelope and the relatively much higher coefficient refractory metal seals in order to avoid or at least better distribute thermally induced mechanical stresses. The production of such glass-to-metal seals is complex, time consuming and expensive and, even in the most favorable light, the multiple glass composition seals constitute only a partially effective expedient since they neither resolve the condensation problem nor do they avoid the need for protection of the metal seal elements from external oxidation thereof.
In accordance with the construction of the present invention, however, these problems have either been essentially completely resolved or at least substantially ameliorated.
It is a principal object of the invention to provide a novel glass-to-metal seal construction.
It is another object of the invention to provide a glass-to-metal seal construction suitable for use in the fabrication of discharge devices of the compact arc discharge lamp or gas discharge laser type wherein the need for external cooling of the seal-containing portions of the device in order to avoid excessive external oxidation of refractory metal seal elements is relieved.
It is another object of the invention to provide a glass-to-metal seal construction suitable for use in the fabrication of discharge devices of the compact arc discharge lamp or gas discharge laser type wherein temperatures above the condensation temperatures of condensible components contained within the envelope of the device may be maintained at the seal-containing portions thereof without excessive external oxidation of the refractory metal seal elements.
It is another object of the invention to provide a glass-to-metal seal construction suitable for use in the fabrication of devices of the compact arc discharge lamp or gas discharge laser type wherein external cooling of the end portion of said seal construction may be employed without substantially reducing the temperature within the galss envelope of the device.
It is another object of the invention to provide a novel direct quartz-to-metal seal construction suitable for use in the fabrication of devices of the compact arc discharge lamp or gas discharge laser type wherein mechanical stresses due to thermal coefficient of expansion mismatches between the metallic and quartz elements of the construction are at least partially compensated.
It is still another object of the invention to provide a glass-to-metal seal construction wherein the glass elements thereof are composed of a single glass composition.
Other objects and advantages of the invention will in part be obvious and will in part appear hereinafter.